1. Field of the Invention
The present invention relates to a method of producing a ferroelectric thin film and, more particularly, to a method of producing a high-purity LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) thin film.
2. Description of the Prior Art
It is well known that LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) is a ferroelectric material having a high melting point and a high Curie temperature and that it has a large electromechanical coupling coefficient as compared with other ferroelectric materials. Because of these properties, the bulk single crystal of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) has been put to practical use as a material for surface acoustic wave (SAW) devices. By utilizing the electro-optical effect and nonlinear optical effect of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1), wide application of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) has been developed for use in optical devices, for example, as a substrate material for optically integrated circuit material including an optical waveguide, optical modulator, optical coupler and wavelength transducer. Attempts, have also been made for the application of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) in various optical IC sensors which utilize the change in the refractive index caused by a change in the environmental or external factors such as stress and temperature. In addition, LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) exhibits a special optical effect when impurities such as Fe are added thereto, the impurities greatly change the refractive index of the compound under conditions of light irradiation. The application of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) to an optical memory or a three dimensional hologram by utilizing the above-described phenomenon has also been investigated.
At the first stage, a wafer for developing devices for the above-described applications of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) is cut from a bulk single crystal which has been produced by a pulling method and which has an appropriate crystalline orientation and a thickness of several hundred .mu.m.
In the case where a wafer is cut from a bulk single crystal which has been obtained by a pulling method, it is only a region of several tens of .mu.m thick from the surface which is important for the actual functioning of the device. Therefore, if it is possible to produce a device of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1) in the form of a thin-film device, great benefit in terms of effectiveness can be expected from the points of view of properties, materials, cost and productivity. In the present state of the art, however, no technique of producing such a thin film has been adequately established.
As a thin-film forming method of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1), sputtering, liquid phase epitaxy and CVD have conventionally been reported. Among these, sputtering is used most widely. It has been reported that according to this method, a thin-film twin crystal is hetero-epitaxially grown on a single crystal substrate of sapphire (Z face and R face), rock crystal (Z face), magnesium oxide (&lt;111&gt;face), etc. (Japanese Patent Publication Nos. 29280/1983 and 5560/1984).
This is a method of sputtering a target of a powder or a sintered powder having the same composition as the intended material so as to form an oxide thin film on a substrate. A method of sputtering a metal or alloy target in an oxygen gas atmosphere is also used. Since inert gas such as Ar gas is used as a sputtering gas in both of the above-described methods, there is a strong possibility of Ar, or the like, mixing with the thin film as impurities, so that it is difficult to produce a thin film having the desired properties at a comparatively low temperature.
In the case of forming a thin-film single crystal by hetero-epitaxial growth on a substrate which has a large lattice mismatching, it is important that the crystal lattice has a lattice constant approximating that of the substrate. For this purpose, it is necessary to change the composition of the thin film which is being formed. In order to mitigate the lattice mismatching, it is necessary to continuously change the composition of the film or laminating films which have different compositions in multi-layers, and in order to obtain the optimal conditions to diligently control the materials depending on the results by trial and error in many experiments.
Another method is to simultaneously deposit oxide materials from an evaporating source which correspond to the respective components of a thin film by electron beam evaporation or resistance heating. Since the evaporation rate is controlled by varying the heating temperatures for the respective components, it is comparatively easy to control the composition of the film. By this method, however, since an oxide is used as a material, the oxygen component of the materials is lacking because of the difference in the vapor pressure at the atomic level between a metal element and an oxygen element, thereby making it difficult to form a thin film having a uniform composition. As a countermeasure, a method of simultaneously evaporating by heating a plurality of single element components in an oxygen gas atmosphere in place of oxide materials has been proposed. In the case of LiNb.sub.1-x Ta.sub.x O.sub.3 (0.ltoreq.x.ltoreq.1), however, since it is impossible to efficiently introduce oxygen into the film, the film formation in an oxygen atmosphere is difficult.